A motor control circuit uses a control transformer to step down 480V to 120V for the control power. The control transformer has a primary overcurrent protective device rated at 2.5A. The transformer's primary impedance is 5%. According to NEC 430.72(C), what is the maximum rating permitted for the primary overcurrent protective device when secondary overcurrent protection is not provided?
- A6.25A
- B15A
- C2.5A
- D12.5A
Show answer & explanationAnswer & explanation
Correct answer: A. 6.25A
NEC 430.72(C)(1) for control transformers without secondary protection states that the primary overcurrent device shall be rated or set at not more than 250% of the rated primary current of the transformer. In this case, the rated primary current is given as 2.5A. So, 2.5A * 2.50 = 6.25A. The transformer's impedance is irrelevant for this calculation, as it applies when determining inrush current for sizing, but not for the direct application of 430.72(C)(1).
Why the other options are wrong
- B. This is a common standard fuse/breaker size, but it exceeds the calculated maximum permissible rating.
- C. This is the primary current, not the maximum allowed overcurrent protection rating.
- D. This value (500% of primary current) is not a standard multiplier for control transformers without secondary protection.
Control Transformer Primary OCPD (No Secondary Protection)
According to NEC 430.72(C)(1), if a control transformer's secondary is not separately protected, its primary overcurrent protective device must be rated or set at not more than 250% of the transformer's rated primary current.
- Applies when no secondary OCPD is present.
- Maximum primary OCPD is 250% of primary current.
- Primary current is typically very low for control transformers.
- Ensures protection of the transformer itself.
Memory trick: No secondary watch, primary gets a 'two-and-a-half' stretch.